Government forces have since joined the battle by shelling Zaidi fighter positions.
 
The on-off uprising led by the Zaidi fighters against the Yemeni government has claimed thousands of lives since 2004.
 
Zaidi minority
 
An offshoot of Shia Islam, the Zaidis are a minority in mainly Sunni Yemen but form the majority in the northwest.
 
Their stated aim is to reinstate the Zaidi Imamate that ruled northern Yemen until the 1962 revolution.
 
The fighters reject the rule of Ali Abdullah Saleh, the Yemeni president, as illegitimate, although Saleh is himself a Zaidi.
 
Saleh has led Yemen since the unification of north and south in 1990. Before that he led North Yemen for 12 years.